The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts:
-ansi-std=c90. In C++ mode, it is equivalent to -std=c++98. This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++ (when compiling C++ code), such as the asm and typeof keywords, and predefined macros such as unix and vax that identify the type of system you are using. It also enables the undesirable and rarely used ISO trigraph feature. For the C compiler, it disables recognition of C++ style ‘//’ comments as well as the inline keyword.
The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue to work despite -ansi. You would not want to use them in an ISO C program, of course, but it is useful to put them in header files that might be included in compilations done with -ansi. Alternate predefined macros such as __unix__ and __vax__ are also available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that, -Wpedantic is required in addition to -ansi. See Warning Options.
The macro __STRICT_ANSI__ is predefined when the -ansi option is used. Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things.
Functions that are normally built in but do not have semantics defined by ISO C (such as alloca and ffs) are not built-in functions when -ansi is used. See Other built-in functions provided by GCC, for details of the functions affected.
-std=The compiler can accept several base standards, such as ‘c90’ or ‘c++98’, and GNU dialects of those standards, such as ‘gnu90’ or ‘gnu++98’. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that do not contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the asm and typeof keywords, but not other GNU extensions that do not have a meaning in ISO C90, such as omitting the middle term of a ?: expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style ‘//’ comments, while -std=gnu99 -Wpedantic does not.
A value for this option must be provided; possible values are
c90’c89’iso9899:1990’-ansi for C code. iso9899:199409’c99’c9x’iso9899:1999’iso9899:199x’c9x’ and ‘iso9899:199x’ are deprecated. c11’c1x’iso9899:2011’c1x’ is deprecated. gnu90’gnu89’gnu99’gnu9x’gnu9x’ is deprecated. gnu11’gnu1x’gnu1x’ is deprecated. c++98’c++03’-ansi for C++ code. gnu++98’gnu++03’-std=c++98. This is the default for C++ code. c++11’c++0x’c++0x’ is deprecated. gnu++11’gnu++0x’-std=c++11. The name ‘gnu++0x’ is deprecated. c++14’c++1y’c++1y’ is deprecated. gnu++14’gnu++1y’-std=c++14. The name ‘gnu++1y’ is deprecated. c++1z’gnu++1z’-std=c++1z. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. -fgnu89-inline-fgnu89-inline tells GCC to use the traditional GNU semantics for inline functions when in C99 mode. See An Inline Function is As Fast As a Macro. Using this option is roughly equivalent to adding the gnu_inline function attribute to all inline functions (see Function Attributes). The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for inline when in C99 or gnu99 mode (i.e., it specifies the default behavior). This option is not supported in -std=c90 or -std=gnu90 mode.
The preprocessor macros __GNUC_GNU_INLINE__ and __GNUC_STDC_INLINE__ may be used to check which semantics are in effect for inline functions. See Common Predefined Macros.
-aux-info filename
Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (‘I’, ‘N’ for new or ‘O’ for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (‘C’ or ‘F’, respectively, in the following character). In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration.
-fallow-parameterless-variadic-functionsAlthough it is possible to define such a function, this is not very useful as it is not possible to read the arguments. This is only supported for C as this construct is allowed by C++.
-fno-asmasm, inline or typeof as a keyword, so that code can use these words as identifiers. You can use the keywords __asm__, __inline__ and __typeof__ instead. -ansi implies -fno-asm. In C++, this switch only affects the typeof keyword, since asm and inline are standard keywords. You may want to use the -fno-gnu-keywords flag instead, which has the same effect. In C99 mode (-std=c99 or -std=gnu99), this switch only affects the asm and typeof keywords, since inline is a standard keyword in ISO C99.
-fno-builtin-fno-builtin-function
__builtin_’ as prefix. See Other built-in functions provided by GCC, for details of the functions affected, including those which are not built-in functions when -ansi or -std options for strict ISO C conformance are used because they do not have an ISO standard meaning. GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to alloca may become single instructions which adjust the stack directly, and calls to memcpy may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library. In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with -Wformat for bad calls to printf when printf is built in and strlen is known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with ‘__builtin_’. If a function is named that is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n)) #define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhosted-fbuiltin. A hosted environment is one in which the entire standard library is available, and in which main has a return type of int. Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding. -ffreestanding-fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at main. The most obvious example is an OS kernel. This is equivalent to -fno-hosted. See Language Standards Supported by GCC, for details of freestanding and hosted environments.
-fopenacc#pragma acc in C/C++ and !$acc in Fortran. When -fopenacc is specified, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.0 http://www.openacc.org/. This option implies -pthread, and thus is only supported on targets that have support for -pthread. Note that this is an experimental feature, incomplete, and subject to change in future versions of GCC. See https://gcc.gnu.org/wiki/OpenACC for more information.
-fopenmp#pragma omp in C/C++ and !$omp in Fortran. When -fopenmp is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.0 http://www.openmp.org/. This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd. -fopenmp-simd#pragma omp in C/C++ and !$omp in Fortran. Other OpenMP directives are ignored. -fcilkplus-fcilkplus is specified, enable the usage of the Cilk Plus Language extension features for C/C++. The present implementation follows ABI version 1.2. This is an experimental feature that is only partially complete, and whose interface may change in future versions of GCC as the official specification changes. Currently, all features but _Cilk_for have been implemented. -fgnu-tm-fgnu-tm is specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature. For more information on GCC's support for transactional memory, See The GNU Transactional Memory Library.
Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions).
-fms-extensionsIn C++ code, this allows member names in structures to be similar to previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
}; Some cases of unnamed fields in structures and unions are only accepted with this option. See Unnamed struct/union fields within structs/unions, for details.
Note that this option is off for all targets but x86 targets using ms-abi.
-fplan9-extensionsThis enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef. See Unnamed struct/union fields within structs/unions, for details. This is only supported for C, not C++.
-trigraphs-ansi option (and -std options for strict ISO C conformance) implies -trigraphs.
-traditional-traditional-cpp-E switch. The preprocessor continues to support a pre-standard mode. See the GNU CPP manual for details. -fcond-mismatch-flax-vector-conversions-funsigned-charchar be unsigned, like unsigned char. Each kind of machine has a default for what char should be. It is either like unsigned char by default or like signed char by default.
Ideally, a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But many programs have been written to use plain char and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default.
The type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two.
-fsigned-charchar be signed, like signed char. Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, the option -fno-signed-char is equivalent to -funsigned-char.
-fsigned-bitfields-funsigned-bitfields-fno-signed-bitfields-fno-unsigned-bitfieldssigned or unsigned. By default, such a bit-field is signed, because this is consistent: the basic integer types such as int are signed types.
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https://gcc.gnu.org/onlinedocs/gcc-5.4.0/gcc/C-Dialect-Options.html